Field of the Invention
The present invention relates to a substrate for an organic light-emitting device, a method of fabricating the same and an organic light-emitting device including the same, and more particularly, a substrate for an organic light-emitting device which not only improves light extraction efficiency but also has superior productivity and fabrication efficiency, a method of fabricating the same and an organic light-emitting device including the same.
Description of Related Art
In general, light-emitting devices can be generally divided into organic light-emitting devices in which a light-emitting layer is formed from an organic matter and inorganic light-emitting devices in which a light-emitting layer is formed from an inorganic matter. An organic-light-emitting diode used in organic light-emitting devices is a self-emitting element which generates light using energy emitted from excitons that are generated through the recombination of electrons injected through a cathode and holes injected through an anode. Such organic light-emitting devices have a variety of advantages, such as, low-voltage driving, self-emission, a wide viewing angle, a high resolution, natural color reproduction and rapid response.
Recently, active studies are underway in order to apply organic light-emitting devices to a variety of devices, such as portable information devices, cameras, watches, office equipment, information display windows of vehicles, televisions (TVs), displays, or illumination systems.
Approaches for improving the luminous efficiency of organic light-emitting devices include an approach of improving the luminous efficiency of a material that constitutes a light-emitting layer and an approach of improving the light extraction efficiency at which light generated from the light-emitting layer is extracted.
The light extraction efficiency depends on the refractive indices of the layers which form an organic light-emitting device. In a typical organic light-emitting device, when a ray of light generated from the light-emitting layer is emitted at an angle greater than a critical angle, the ray of light is totally reflected at the interface between a higher-refractivity layer which could be a transparent electrode layer and a lower-refractivity layer which could be a substrate. This consequently lowers the light extraction efficiency, thereby lowering the overall luminous efficiency of the organic light-emitting device, which is problematic.
More specifically, only about 20% of light generated from an organic light-emitting diode is emitted to the outside and about 80% of the light is lost by a waveguide effect originating from the difference in the refractive index between a glass substrate and an organic light-emitting diode which includes an anode, a hole injection layer, a hole transport layer, an emissive layer, an electron transport layer and an electron injection layer, as well as by the total internal reflection originating from the difference in the refractive index between the glass substrate and the air. Here, the refractive index of the internal organic light-emitting layer ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO) which is generally used for the anode is about 1.9. Since the two layers have a very small thickness ranging from 200 to 400 nm and the refractive index of the glass used for the glass substrate is about 1.5, a planar waveguide is thereby formed inside the organic light-emitting device. It is calculated that the ratio of the light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate is about 1.5 and the refractive index of the ambient air is 1.0, when the light is directed outward from the inside of the glass substrate, a ray of the light having an angle of incidence greater than a critical angle is totally reflected and is trapped inside the glass substrate. The ratio of the trapped light is about 35%. Therefore, only about 20% of the generated light is emitted to the outside.
In order to overcome this, a variety of methods for improving light extraction efficiency has been studied. For example, a planarization layer having an intermediate refractive index, as an antireflection film, is provided between a glass substrate and a light-emitting structure, or a partition wall in which white particulates or transparent particulates is dispersed in polymer, as an optical waveguide, is provided on a substrate, the refractive index of the transparent particulates being different from that of the polymer.
A typical technology for light extraction is to coat a substrate with a light-scattering layer including scattering particles. That is, metal oxide particles are contained in a matrix to act as scattering particles, whereby a light scattering effect can be expected at the boundaries between the metal oxide particles and the matrix and from the difference in the refractive index therebetween.
The light-scattering layer is fabricated typically by a wet coating method, such as spin coating or bar coating. However, the wet coating causes some problems.
It is difficult to uniformly disperse the metal oxide particles in the matrix by the wet coating method. According to the characteristics of the wet coating, the volume of the liquid matrix decreases by ⅕ to 1/20 during the baking process. Then, some of the metal oxide particles remaining in the matrix protrude from the surface of the matrix when the volume of the matrix decreases, thereby increasing the surface roughness of the matrix. This consequently has an adverse effect on the characteristics of the organic light-emitting device.
In addition, in the related art, after fabrication of a light extraction layer in which the scattering particles are disposed inside the matrix by the above-mentioned wet coating method, a planarization layer which reduces the surface roughness of the matrix layer is disposed on the matrix for the reliability of an organic light-emitting device. Afterwards, a transparent electrode, an organic light-emitting layer and a metal electrode are sequentially deposited on the resultant structure by a dry process.
Since the deposition process for the light extraction layer and the deposition process for the organic light-emitting diode are carried out by different processes in the related art, some handling is required between the two processes. This handling between the processes causes a problem to the organic light-emitting device, the efficiency of which is significantly sensitive to a minute defect.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.